Single crystal silicon, which is the starting material in most processes for fabricating semiconductor electronic components, is commonly prepared according to the so-called Czochralski process. In this process, polycrystalline silicon, or polysilicon, is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and a single crystal ingot is grown by relatively slow extraction. After formation of a neck is complete, decreasing the pulling rate and/or the melt temperature enlarges the diameter of the crystal until a desired or target diameter is reached. The generally cylindrical main body of the crystal, which has an approximately constant diameter, is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter is gradually reduced to form an end-cone. Typically, increasing the crystal pull rate and heat supplied to the crucible forms the end-cone. When the diameter becomes small enough, the crystal is then separated from the melt.
As in known in the art, molten silicon (at about 1420 degrees Celsius (° C.)) will dissolve the surface of a silica (SiO2) crucible containing the melt. Some of the dissolved silica evaporates from the surface of the melt as SiO (silicon monoxide) while some of the dissolved silica becomes incorporated into the growing crystal. The remainder of the dissolved silica remains in the melt. In this manner, the crucible containing the silicon melt acts as a source of oxygen that is found in silicon crystals grown by the conventional Czochralski technique.
Oxygen in the silicon crystal may have both favorable and unfavorable effects. In the various heat treatment processes during the manufacture of various electrical devices, the oxygen in the crystal may cause crystal defects such as precipitates, dislocation loops, and stacking faults or it may cause electrically active defects resulting in devices with inferior performance characteristics. The solid solution of oxygen in the crystal, however, increases the mechanical strength of silicon wafers and the crystal defects may improve the yield of conforming products by entrapping contaminants of heavy metals. Accordingly, oxygen content of the silicon crystal is an important factor for product quality that should be carefully controlled in accordance with the ultimate application for the silicon wafers.
The oxygen concentration in a conventional silicon crystal grown under Czochralski conditions prevalent in the industry varies along the length of the crystal. For example, the concentration is typically higher at the seed end than in the middle and/or at the bottom or tang end of the crystal. In addition, oxygen concentration typically varies along the radius of a cross-sectional slice of the crystal.
To address this oxygen control problem, attention has been given to the use of magnetic fields to stabilize convective flows in metal and semiconductor melts for controlling oxygen concentration and radial distribution to remove dopant striation, etc. For example, the Lorentz force generated by magnetic fields in a conductive melt may be used to dampen natural convective flow and turbulence. There are three conventional types of magnetic field configurations used to stabilize convective flows in conductive melts, namely, axial, horizontal, and cusped.
The axial (or vertical) magnetic field configuration (e.g., see FIG. 1) generates a magnetic field parallel to the crystal-growth direction. In FIG. 1, a magnet coil 21, shown in cross-section, supplies a magnetic field to a crucible 23. As shown, the crucible 23 contains a silicon melt 25 from which a crystal 27 is grown. This configuration has the advantages of relatively simple setup and axial symmetry. But the axial magnetic field configuration destroys radial uniformity due to its dominant axial field component.
In the horizontal (or transverse) magnetic field configuration (e.g., see FIG. 2), two magnetic poles 29 are placed in opposition to generate a magnetic field perpendicular to the crystal-growth direction. The horizontal configuration has the advantage of efficiency in damping a convective flow at the melt surface. But its non-uniformity both axially and radially and the complex and bulky setup make it difficult to apply the horizontal magnetic field configuration in large diameter Czochralski growth processes.
The cusped magnetic field configuration (e.g., see FIG. 3) is designed to overcome the deficiencies of the axial and horizontal magnetic field configurations. A pair of coils 31, 33 (e.g., Helmholtz coils) placed coaxially above and below a melt-solid interface and operated in an opposed current mode generates a magnetic field that has a purely radial field component near the melt surface and a purely axial field component near the center of the melt. In this manner, the cusped magnetic field configuration attempts to preserve the rotational symmetry at the interface between the melt and the crystal. However, the effectiveness of the cusped magnetic field is decreased at the center of the melt. Furthermore, since the cusp position remains at the melt surface, axial uniformity within the melt is gradually reduced and eventually disappears as the melt depth is decreased towards the end of crystal growth.
Accordingly, improved control of the crystal growth process is desired to address the disadvantages of these conventional magnetic field configurations.